The present invention relates to a magnetic resonance imaging apparatus for obtaining the image of a subject by utilizing NMR (Nuclear Magnetic Resonance) phenomenon, and an RF coil to be used therefor.
Various RF coils have been used for the magnetic resonance imaging apparatus. Among others, a surface coil has been often used which is obtained by annularly forming a conductor to be a coil face. In addition to circular and rectangular surface coils, an 8-shaped coil has been often used.
The 8-shaped coil is shown in FIG. 13 and has been often used for imaging by so-called vertical field MRI in which the direction of a static magnetic field is substantially vertical to the body axis of the subject. The 8-shaped coil described herein indicates a general coil in which a conductor in a central portion (hereinafter referred to as a "central conductor") 100 has a sensitivity to an MR signal of a subject region and a current return path is provided to bypass the outside of the central conductor 100. A static magnetic field B.sub.0 direction is a z direction.
FIG. 14 shows another example of the 8-shaped coil according to the prior art. The 8-shaped coil shown in FIG. 14 does not have a wholly square 8-shape shown in FIG. 13 but has a smooth 8-shape. A coil having a shape shown in FIG. 15 is also included in the 8-shaped coil.
FIG. 16 shows the positional relationship between the subject and the 8-shaped coil. In a case where an ordinary circular or rectangular shaped surface coil is put below the medulla spinals of a lying subject to carry out imaging of the medulla spinals of the subject by the vertical field MRI, a sensitivity magnetic field direction is the z direction which is parallel with the direction of a static magnetic field and has no effective MRI signal. In the 8-shaped coil, the direction of the magnetic field where the central conductor has a sensitivity is set to a direction (y direction) orthogonal to a transit direction (x direction) of the central conductor and a region having the sensitivity is placed in the vicinity of the central conductor. Thus, if the 8-shaped coil is provided in such a manner that the transit direction of the central conductor is parallel with the transit direction of the medulla spinals (x direction), a magnetic field sensitivity region in a direction along the medulla spinals can be obtained. In this case, the magnetic field sensitivity direction is orthogonal to the static magnetic field. In this direction, an effective MRI signal is obtained, a long sensitivity region is provided in the direction of the medulla spinals and an RF coil (surface coil) useful for a vertical field MRI apparatus is obtained.
However, the RF coil according to the prior art has a small effective sensitivity region in a transverse direction (y direction) as is apparent from FIG. 16. For this reason, there is a problem in that the use of the RF coil is restricted to only diagnosis of a slender subject portion such as the medulla spinals.
In a case where an 8-shaped coil having a sensitivity concentrated only in the vicinity of the central conductor is used to carry out imaging of the medulla spinals, for example, a glaring image is obtained in a portion close to the body surface of the subject and the sensitivity is rapidly lowered in a portion greatly apart from the body surface. According to such an uneven sensitivity, there are problems in that S/N tends to be lacking in a certain deep region and a final image is diagnosed with difficulty.
As one of well-known techniques, a plurality of surface coils having sensitivity magnetic field directions different from each other are combined to form a surface QD (Quadrature Detection) coil having higher S/N. In general, this technique has been often used in an MRI apparatus in which the direction of a static magnetic field B.sub.0 is parallel with the direction of the body axis of a subject as shown in FIG. 17.
In a case where the surface QD coil shown in FIG. 17 is used for the vertical field MRI apparatus, the following problem arises. More specifically, a rectangular coil 110 shown in FIG. 17 has no sensitivity to an MRI signal sent from a medulla spinals portion. Therefore, the S/N cannot be enhanced.
Therefore, it is also supposed that a surface QD coil is constituted by overlapping two 8-shaped coils 130 and 140 orthogonally to each other as shown in FIG. 18.
Also in this case, however, the following problem arises. More specifically, the surface QD coil can have high S/N only in a region 150 in the vicinity of a portion where both central conductors intersect. Consequently, it is impossible to keep a region which is long in the x direction and has high S/N. For this reason, in a case where the position of an affected part is precisely grasped in advance, a part of the medulla spinals can be observed with high quality of image. However, the long region of the medulla spinals cannot be observed. Similarly, high S/N can be obtained in a small visual field in the y-direction. Therefore, the use is limited.